Circuit
Dual-voltage 220V / 380V input systems
Voltage detection, boost/doubler switching, relay control, comparator logic and wrong-voltage patterns.
What this fault means
Dual-voltage welders use an input detection and switching circuit to work on 220V or 380V. Some designs use a doubler or boost path on 220V and a different rectifier/filter state on 380V. If the detection or relay path fails, the DC bus can be wrong and the welder may not start or may damage capacitors.
Input conversion flow
AC input sampled
↓
Voltage divider / comparator
↓
LM358 or logic decision
↓
Optocoupler / relay driver
↓
Boost or doubler switching
↓
DC bus reaches expected range
↓
Auxiliary and PWM stages start
Checks
| Check | Normal / Expected | Abnormal Meaning |
|---|---|---|
| Input sampling | Correct scaled voltage | Resistor divider or connector fault |
| Comparator output | Changes with input voltage | LM358/reference fault |
| Relay drive | Energizes in correct mode | Driver transistor or relay coil fault |
| DC bus | Expected for selected input | Boost/doubler not switching |
| Bulk capacitors | No bulge or leakage | Wrong voltage or overvoltage damage |
Repair warning
Wrong dual-voltage switching can create overvoltage on filter capacitors. Never assume the board is safe just because the machine powers on.